Replace character's position in a string

Question

In PHP, how can you replace the second and third character of a string with an X so string would become sXXing?

The string\'s

Answer 1

function mb_substr_replace($string, $replacement, $start, $length=0)
{
    return mb_substr($string, 0, $start) . $replacement . mb_substr($string, $start+$length);
}

same as above, but standardized to be more like substr_replace (-substr- functions usually take length, not end position)